Multiple effect evaporators are used in industries where a particular process stream must be concentrated by evaporation to rid the liquid of a substantial amount of its water content to make it more manageable or usable in downstream processes. A typical example is in the practice of conventional continuous chemical pulp production.
Multiple effects, or stages, are now used to minimize the energy input required to evaporate or boil off undesirable water content. The total evaporation achieved in these systems is approximately the number of effects times the energy input to the first effect.
In conventional, multiple effect evaporator systems, a high level (temperature) energy source is input only to the first effect in the series, thereby evaporating water at a lower energy (temperature) level and providing heat for further evaporation in the next effect of the series. This process is repeated until water evaporated in the last of these effects is at its lowest practical energy level. This low level energy in the form of vapor is condensed in a surface condenser using a cooling water source which serves as the ultimate heat sink for the system and maintains the condensing pressure on the evaporator effects preceding it. However, the heat of condensation added to the cooling water takes away its capacity for reuse as a coolant.
Since the cooling water must be provided on a continuous basis, either new cooling water must be provided, while rejecting the heated cooling water or, the heated water can be reused but only after it has gone through a cooling process where the heat absorbed in the surface condenser connected to the multiple effect evaporators is rejected to a cooling air stream.
Previous inventions have sought to simplify the process by utilizing an evaporative cooling surface condenser, whereby the low level energy vapor from the heat evaporator effect is condensed on a heat transfer surface, giving up its heat of condensation to a cooling fluid flowing as a falling film on the outside of the heat transfer surface. At the same time heat is being added to the cooling fluid, the same fluid is being evaporatively cooled by the flow of cooling air around the spaces between the heat transfer surface members. After leaving the heat transfer surface and entering a collection sump underneath, the cooling water is ready for reuse as a coolant while the heat from the last effect of the evaporator has eventually been rejected into a cooling air stream.